Explanation:
Saturn's ragged moon Rhea has one of the oldest surfaces known.
Estimated as changing little in the past billion years,
Rhea shows
craters
so old they no longer appear round – their
edges have become compromised by more recent cratering.
Like Earth's Moon,
Rhea's rotation is locked on Saturn, and the
above image shows part of
Rhea's surface that always faces Saturn.
Rhea's leading surface is more highly cratered than its trailing surface.
Rhea is composed mostly of water-ice but is thought to include about 25 percent rock and metal.
The above image
was taken by the
robot Cassini spacecraft now orbiting Saturn.
Cassini swooped past
Rhealast month
and captured the
above image
from about 350,000 kilometres away.
Rhea
spans 1,500 kilometres making it Saturn's second largest moon after
Titan.
Several
surface features on Rhea
remain unexplained including large light patches like those seen near the image top.